Between my Xbox 360 and my PS3, my living room is often mistaken for an airstrip. There is always some sort of loud fan whirring away, desperately trying to keep my entertainment centre cool. ExtremeTech has posted a handy guide for quieting down your 360 fan, which I will condense here.
1. Open your 360, thus voiding the precious warranty.
2. Firmly grasp your system fan, ungrasping as your hand swings in a graceful arc towards your trash can.
3. Install new, glowy Whisper fan.
Of course it is much more complicated than that, but it does result in a much quieter Xbox 360, perfect for streaming your legally procured anime episodes. Of course you still have to deal with the loudest optical media drive this side of the Dreamcast, but it's a start.
Hacking Your Xbox 360 Fan [ExtremeTech]